you would need some sort of oscilloscope to measure the O2 output, if it gets weak you will lose serious gas mileage.

I have a chev impala 3.4 too in the family - it was drinking gas like its going out of style - no errors - replaced the o2 sensor and FPR - went right back to 20mpg.

a good starting point would be to check the exhaust for blockage (pre cat failure), o2 sensors gone blind or leaking FPR.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)